Thermosyphon reboiler for the denitrogenation of liquid natural gas

ABSTRACT

Processes are provided for the denitrogenation of a crude LNG stream. A crude LNG stream is expanded and then cooled in a thermosyphon reboiler. The resultant crude LNG stream is introduced into a nitrogen rejection column, wherein the nitrogen content of the LNG is reduced as the liquid flows down the column. A nitrogen-enriched vapor stream is withdrawn from the top of the column, and a first, and optionally a second, nitrogen-diminished liquid stream is withdrawn from the bottom of the column. The first stream may be recovered as a LNG product, and the second stream is passed through the thermosyphon reboiler, cooling the crude LNG stream, while partially vaporizing the second stream. The vaporized second stream is reinjected into the column at a level above the level of withdrawal of the second nitrogen-diminished bottoms LNG stream to provide column boilup. Alternatively, the thermosyphon reboiler is placed within the column.

BACKGROUND OF THE INVENTION

This invention relates to processes for the separation of nitrogen froma liquid natural gas stream comprising nitrogen, methane, and possiblyheavier hydrocarbons.

Crude natural gas is often liquefied to enable storage of largerquantities in the form of liquid natural gas (LNG). Because natural gasmay be contaminated with nitrogen, nitrogen is advantageously removedfrom LNG to produce a nitrogen-diminished LNG product that will meetdesired product specifications. Several methods of effectuating nitrogenremoval from LNG have been disclosed in the prior art.

One simple method for separating nitrogen from a LNG stream is toisentropically expand the crude LNG stream in a turbine and then injectthe stream into a flash separator. The liquid product removed from theflash separator will contain less nitrogen than the crude LNG stream,whereas the vapor product will contain a higher proportion of nitrogen.

A different method is disclosed in U.S. Pat. No. 5,421,165 (“the '165patent”). A process is disclosed wherein crude LNG is isentropicallyexpanded in a turbine and cooled in a reboiler heat exchanger. Thecooled and expanded LNG stream is then passed through a valve, where itundergoes static decompression, prior to its injection into adenitrogenation column. Within the column, nitrogen is stripped from thefalling liquid by the rising vapor, so that the vapor stream exiting thetop of the column is enriched with nitrogen. A liquid LNG stream iswithdrawn from the bottom of the column as a nitrogen-diminishedproduct. Within the column, at a level below the level of injection ofthe LNG feed stream, a liquid stream is withdrawn and passed through theheat exchanger to cool the feed and then reinjected into the column at alevel below that at which it had been withdrawn, to provide boilup tothe column. In effect, the passage of the withdrawn stream through theheat exchanger provides an additional equilibrium stage of separation.

A similar method for separating nitrogen from an LNG stream replaces theturbine driven dynamic decompression with a valve for staticdecompression, such that the expansion takes place isenthalpicallyrather than isentropically. The use of the isentropic expansion in theprocess of the '165 patent allegedly permits greater methane recovery.

Another method for removing nitrogen from an LNG stream is described inU.S. Pat. No. 5,041,149 (“the '149 patent”). This patent discloses amethod of removing nitrogen from a crude natural gas stream by firstcooling the stream and then passing it through a phase separator, toproduce a liquid stream and a vapor stream. The liquid stream is furthercooled and injected into a denitrogenation column. The vapor stream iscondensed and cooled further to produce a second liquid stream, prior toinjection into the denitrogenation column at a higher level than that ofthe first liquid stream. Nitrogen-enriched vapor is removed from the topof the column and used to cool the incoming second liquid stream. Thesump of the column is divided by a baffle, one side of which is filledwith liquid from the lowest tray of the column. This bottoms liquid iswithdrawn and at least partially vaporized in the heat exchanger, whilecondensing the vapor stream from the phase separator, and returned tothe column as a reflux stream to provide boilup. The liquid remaining inthe reflux stream falls to the other side of the baffle in the sump.This liquid reflux is then removed as a nitrogen-diminished productstream, pumped to a higher pressure, warmed and vaporized, and thendynamically expanded to reduce the temperature and pressure of the vaporproduct. Similar to the reboiler heat exchange of the '165 patent, thereflux of the bottoms liquid serves as an additional equilibrium stageof separation.

A disadvantage of these prior art nitrogen separation methods is thatthey each require that the entire liquid flow off of one tray berecycled through the reboiler. Another disadvantage is that they eachare completely dependent upon liquid head in the column to drive theheat exchangers. These characteristics limit the flexibility of thesemethods, as the entire process must be designed to accommodate thislarge amount of flow. A further disadvantage associated with the priorart is that the processes tend to require a large area for the placementof equipment.

Accordingly, it is an object of the present invention to provide aprocess which allows for greater flexibility in the design of theequipment necessary for nitrogen rejection from an LNG stream. Thisgreater flexibility allows for the design of relatively inexpensiveprocess equipment, thus lowering the capital costs associated with theprocess. It is another objective of the present invention to provide anitrogen separation process which can be less costly and can savevaluable space through the elimination of certain equipment required ofthe prior art processes.

BRIEF SUMMARY OF THE INVENTION

The present invention provides an improved process for thedenitrogenation of an LNG stream contaminated by nitrogen. This processallows for economic benefits by permitting a greater flexibility in theprocess design and eliminating the requirement of some equipment.

According to one embodiment of the invented process, a crude LNG streamcomprising between about 1% and 10% nitrogen, and the remainder methaneand heavier hydrocarbons, is expanded in a means for expansion, andcooled in a thermosyphon reboiler. The resultant crude LNG stream isintroduced into a nitrogen rejection column, wherein the nitrogencontent of the LNG is reduced as the liquid flows down the column. Anitrogen-enriched vapor stream is withdrawn from the top of the column,and a first nitrogen-diminished liquid stream is withdrawn from thebottom of the column. This nitrogen-diminished liquid stream may berecovered as a LNG product.

A second nitrogen-diminished liquid stream is also withdrawn from thebottom of the column. This second stream is passed through thethermosyphon reboiler, thus cooling the crude LNG stream, and at leastpartially vaporizing the second stream. The partially vaporized secondstream is reinjected into the column at a level above the level ofwithdrawal of the nitrogen-diminished bottoms LNG stream and below thelevel of introduction of the crude LNG feed stream to provide columnboilup.

In an alternative embodiment of the invented process, the first andsecond nitrogen-diminished liquid streams are withdrawn from the columntogether, through the same conduit, and are separated after withdrawal.

In another alternative embodiment of the invented process, thethermosyphon reboiler is placed within the sump of the column so thatonly one nitrogen-diminished liquid stream is withdrawn from the column.

As will become apparent, several variations of these processes arewithin the scope of the invention. For example, in one embodiment, theinitial crude LNG stream is expanded in a dense fluid expander, whichmay be placed either upstream or downstream of the thermosyphonreboiler. A valve may also be placed immediately upstream of thenitrogen rejection column, such that the crude LNG stream is throttledthrough the valve prior to injection into the column.

BRIEF DESCRIPTION OF SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 is a schematic diagram illustrating a first process for removingnitrogen from an LNG stream in accordance with one embodiment of thepresent invention.

FIG. 2 is a schematic diagram illustrating a second process for removingnitrogen from an LNG stream in accordance with one embodiment of thepresent invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention achieves flexibility of design and processeconomic advantages in an LNG denitrogenation operation by using, inpart, a thermosyphon reboiler, the flow through which is driven by thedensity difference between the input and output streams in conjunctionwith the liquid head of the column, rather than solely by the liquidhead of the column, thus permitting a greater flexibility in the overallprocess design. The present invention also permits variability in theamount of fluid flow through the reboiler which further increasesprocess flexibility. Additionally, the processes according to thepresent invention permit the elimination of some equipment, includingcollection trays, nozzles, and large reboilers, that would otherwise berequired of prior art processes, and can therefore achieve theadditional advantages of saving both cost and space.

As will be clarified in the following description, achieving thisflexibility, while allowing for the removal of process equipment and themaintenance of output levels and energy requirements, involves theintroduction of a small thermodynamic inefficiency. However, theflexibility and cost and space savings afforded by the present inventionmore than compensate for this thermodynamic inefficiency, especiallygiven the ease and low expense with which it may be remedied.

The term “nitrogen-enriched stream” is used herein to mean a streamcontaining a higher concentration of nitrogen when compared with aninitial feed stream.

The term “nitrogen-diminished stream” is used herein to mean a streamcontaining a lower concentration of nitrogen when compared with aninitial feed stream.

The term “below” is used herein to mean at a position of lesser height,i.e., closer to the ground.

The term “above” is used herein to mean at a position of greater height,i.e., farther from the ground.

The term “boilup” is used herein to mean vapor which rises up thecolumn.

A preferred embodiment of the invention will now be described in detailwith reference to FIG. 1. The following embodiments are not intended tolimit the scope of the invention, and it should be recognized by thoseskilled in the art that there are other embodiments within the scope ofthe claims.

As set forth in FIG. 1, high-pressure LNG stream 100, typically at apressure of about 700 psia, containing from about 1 mol % to about 10mol % nitrogen, and the remainder methane and possibly heavierhydrocarbons, is expanded via means for expanding the LNG stream 102 toproduce lower-pressure LNG stream 104. The expansion is preferablyperformed isentropically, and the means for expanding the LNG stream ispreferably a dense fluid expander (also known as a hydraulic turbine),but may also be a valve or other known means for expanding a fluid.Lower-pressure LNG stream 104 is cooled in thermosyphon reboiler 106 toproduce cooled, expanded LNG stream 108. Cooled, expanded LNG stream 108is then substantially isenthalpically expanded through valve 109 andinjected into nitrogen rejection column 150, this injection preferablytaking place at the top of the column. Nitrogen rejection column 150 ispreferably a tray column, but may be a packed column or any other masstransfer device suitable for fractionation. A nitrogen-enriched vaporstream 130 is withdrawn from the top of column 150. By“nitrogen-enriched,” it is herein understood to mean containing a higherconcentration of nitrogen than that of high-pressure LNG stream 100, andwill typically contain more than about 30% N₂ and less than about 70%methane.

A first nitrogen-diminished liquid stream 110 is withdrawn from thebottom of column 150 and may be recovered as a product stream. By“nitrogen-diminished,” it is herein understood to mean containing alower concentration of nitrogen than that of high-pressure LNG stream100. A second nitrogen-diminished liquid stream, reboiler stream 112 isalso withdrawn from the bottom of column 150. The flow rate of reboilerstream 112 is typically between about 15% and about 100% of the flowrate of liquid stream 110. Reboiler stream 112 is at least partiallyvaporized in thermosyphon reboiler 106 to produce partially vaporizedreboiler stream 114, which is then injected into the bottom of column150, below the lowest tray in the case of a tray column, or below thepacking material in the case of a packed column, to provide boilup. Indesign stage, the flow rate of reboiler stream 112 may be adjusted asnecessary to provide different recirculation rates (i.e., the ratio ofthe outlet liquid flow to vapor flow).

In an alternative embodiment, the means for expanding the LNG stream 102may be placed downstream of thermosyphon reboiler 106. In this manner,high-pressure stream 100 is cooled in thermosyphon reboiler 106 prior toundergoing expansion in the means for expanding the LNG stream 102.

In another alternative embodiment, nitrogen-diminished liquid stream 110and reboiler stream 112 may be withdrawn from the bottom of column 150as a single stream through a single conduit. According to thisembodiment, nitrogen-diminished liquid stream 110 would then beseparated from the combined stream and optionally recovered as a productstream. The remaining stream would be reboiler stream 112, and wouldproceed through the thermosyphon reboiler as before.

We note that in each of the described embodiments, valve 109 isoptional, and, in the alternative, cooled LNG stream 108 can be directlyinjected into nitrogen rejection column 150. Where valve 109 is notpresent, the means for expanding the LNG stream 102 is preferably atwo-phase dense fluid expander.

A particularly preferred embodiment is provided wherein a crude LNGstream 100 is substantially isentropically expanded in a dense fluidexpander 102 and cooled in a thermosyphon reboiler 106. This cooled,expanded LNG stream 108 is substantially isenthalpically expandedthrough valve 109 and injected into a nitrogen rejection column 150.Within the column, rising vapor strips the nitrogen from the fallingliquid, and a nitrogen-enriched stream 130 is withdrawn from the top ofthe column. A nitrogen-diminished liquid stream 110 is withdrawn fromthe bottom of the column and may be recovered as a product stream.Reboiler stream 112 is also withdrawn from the bottom of column 150.Reboiler stream 112 is at least partially vaporized in thermosyphonreboiler 106 to produce partially vaporized reboiler stream 114, whichis then injected into the bottom of column 150, below the lowest tray inthe case of a tray column, or below the packing material in the case ofa packed column, to provide boilup. The recirculation rate of thereboiler 106 is preferably at least about 4.

The liquid portion of the partially vaporized reboiler stream 114 mixeswith the liquid from the lowest column stage upon reinjection intocolumn 150 such that the nitrogen-diminished liquid stream 110 is notexclusively the liquid from the bottom stage of the rejection column150, or from the thermosyphon reboiler 106, but rather a mixture ofboth. There is a thermodynamic loss associated with the mixing of theliquid streams to provide the withdrawn nitrogen-diminished stream 110.However, this can easily and cheaply be compensated for by the additionof a stage or stages to the nitrogen rejection column 150.

The recirculation rate for the thermosyphon reboiler may be any desiredrate determined by the geometry of the heat exchanger, and therefore,the ratio of the flow rate of the reboiler stream 112 to the flow rateof the nitrogen-diminished liquid stream 110 can be flexibly defined,and may be easily optimized for the particular process.

An alternative embodiment is illustrated in FIG. 2. As in FIG. 1,high-pressure LNG stream 100, is expanded via means for expanding theLNG stream 102 to produce lower-pressure LNG stream 104; lower-pressureLNG stream 104 is cooled in thermosyphon reboiler 106 to produce cooled,expanded LNG stream 108; and cooled, expanded LNG stream 108 is thensubstantially isenthalpically expanded through valve 109 and injectedinto nitrogen rejection column 150. However, in this embodiment, thethermosyphon reboiler 106 is placed within the sump of the column 150,such that the top of the reboiler 106 is above the height of the liquid.Liquid within the sump is passed through the thermosyphon reboiler 106by means of a pressure gradient established within the reboiler 106,which forces liquid into the bottom of the reboiler 106 and expelsliquid and vapor from the top of the reboiler 106, at a ratio defined bythe recirculation rate. This orientation eliminates the need to withdrawa reboiler stream from, and reinject a reboiler stream into, the column150.

A nitrogen-enriched vapor stream 130 is withdrawn from the top of column150. A nitrogen-diminished liquid stream 110 is withdraw from the bottomof the column and may be recovered as product. As in the embodimentsdescribed above, in the design stage the recirculation rate may beadjusted to any value to provide the desired amount of boilup.

The present invention provides a significant improvement in theadaptability and flexibility of a LNG denitrogenation process throughthe implementation of a hydraulically different process from those ofthe prior art. By permitting a thermosyphon reboiler 106 to assist indriving the flow of the reboiler streams 112/114 rather than relyingexclusively on the column head, and by allowing variability in theselection of the design recirculation rate, greater flexibility ofdesign is permitted. This flexibility can lead to a smaller capitalexpense at the remediable cost of a minor thermodynamic loss. Forexample, by altering the recirculation rate (and, consequently, the flowthrough the reboiler), the reboiler and piping requirements associatedwith the process can be adjusted to minimize capital expenditures. Theuse of a thermosyphon rebolier for the denitrogenation of an LNG streamcan also lead to improvements in the controllability of the overallprocess.

In both the internal and external thermosyphon processes of the presentinvention, there is no need for a liquid collection tray below thedistillation section of the column, which would be required were all ofthe liquid coming off of a tray to pass through a reboiler.Additionally, when the internal thermosyphon process of the presentinvention is used, the nozzles required for withdrawal of the recyclestream are also eliminated. Moreover, by placing the thermosyphonreboiler inside of the column, valuable space can be saved because thereis no longer a need to dedicate space outside of the column to thereboiler and associated piping. When the external thermosyphon reboileris used, space may still be saved, due to the simplified piping, smallerrequired heat transfer surface area, and small footprint associated withthermosyphon reboilers as compared to other reboiler types.

EXAMPLES Example 1

To more particularly demonstrate some of the important differencesbetween the process of the present invention and the prior art, processsimulations of the entire natural gas liquefaction process were run,using an ASPEN process simulator, comparing two embodiments of theinvention (“current process”) with the process disclosed in the '165patent. The comparison basis is an equal LNG production and a satisfiedfuel balance (the amount of LNG product flash required to drive a gasturbine driving the process). The respective reference numerals used inthis example refer to FIG. 1, as described above, and the '165 patent(see, e.g., FIG. 1 therein).

The Current Process

A. Recirculation rate: 2.9

With reference to FIG. 1, following expansion in dense fluid expander102, low pressure LNG stream 104, at a flow rate of 125,470 lbmol/hr, apressure of 71.78 psia, a temperature of −242.9° F., and containing2.96% N₂, 95.47% methane, 1.10% C₂ hydrocarbons, and 0.47% heavierhydrocarbons, is cooled in thermosyphon reboiler 106 to produce cooled,expanded LNG stream 108 at a temperature of −252.5° F. and a pressure of64.52 psia. Cooled, expanded stream 108 is throttled through valve 109and introduced into a denitrogenation column 150 comprising 6 trays, ata pressure of 18 psia. An overhead vapor stream 130 is withdrawn fromthe top of the column 150 at a flow rate of 8,141 lbmol/hr, and contains31.06% N₂, 68.94% methane, and trace amounts of heavier hydrocarbons, ata pressure of 18 psia and a temperature of −261.9° F. Bottoms stream 110is withdrawn from the column 150 at a flowrate of 117,329 lbmol/hr, apressure of 19.45 psia, a temperature of −256.8° F., and contains 1.01%N₂, 97.31% methane, 1.17% C₂ hydrocarbons, and 0.51% heavierhydrocarbons. A reboiler stream 112 is withdrawn from the column 150 ata flow rate of 17,704 lbmol/hr, a temperature of −256.8° F., a pressureof 19.74 psia, and contains 1.01% N₂, 97.31% methane, 1.17% C₂hydrocarbons, and 0.51% heavier hydrocarbons. The reboiler stream 112 ispassed through the thermosyphon reboiler 106, where it is partiallyvaporized to produce vaporized reboiler stream 114. Vaporized reboilerstream 114, which is at a temperature of −252.7° F., a pressure of 19.45psia, and has a vapor fraction of 25.3%, is injected below the bottomtray of column 150 to provide boilup. This liquefaction process requiresapproximately 229 MW of power.

B. Recirculation rate: 26.0

With reference to FIG. 1, following expansion in dense fluid expander102, low pressure LNG stream 104, at a flow rate of 125,474 lbmol/hr, apressure of 71.84 psia, a temperature of −242.9° F., and containing2.96% N₂, 95.47% methane, 1.10% C₂ hydrocarbons, and 0.47% heavierhydrocarbons, is cooled in thermosyphon reboiler 106 to produce cooled,expanded LNG stream 108 at a temperature of −253.1° F. and a pressure of64.59 psia. Cooled, expanded stream 108 is throttled through valve 109and introduced into a denitrogenation column 150 comprising 6 trays, ata pressure of 18 psia. An overhead vapor stream 130 is withdrawn fromthe top of the column 150 at a flow rate of 8,121 lbmol/hr, and contains31.54% N₂, 68.46% methane, and trace amounts of heavier hydrocarbons, ata pressure of 18 psia and a temperature of −262.0° F. Bottoms stream 110is withdrawn from the column 150 at a flowrate of 117,353 lbmol/hr, apressure of 19.45 psia, a temperature of −256.7° F., and contains 0.98%N₂, 97.34% methane, 1.17% C₂ hydrocarbons, and 0.51% heavierhydrocarbons. A reboiler stream 112 is withdrawn from the column 150 ata flow rate of 117,353 lbmol/hr, a temperature of −256.7° F., a pressureof 19.74 psia, and contains 0.98% N₂, 97.34% methane, 1.17% C₂hydrocarbons, and 0.51% heavier hydrocarbons. The reboiler stream 112 ispassed through the thermosyphon reboiler 106, where it is partiallyvaporized to produce vaporized reboiler stream 114. Vaporized reboilerstream 114, which is at a temperature of −254.8° F., a pressure of 19.45psia, and has a vapor fraction of 3.7%, is injected below the bottomtray of column 150 to provide boilup. This liquefaction process alsorequires approximately 229 MW of power.

Prior Art Process

With reference to FIG. 1 of the '165 patent, following expansion inturbine 21, semidecompressed LNG stream 22, at a flow rate of 125,451lbmol/hr, a pressure of 71.76 psia, a temperature of −242.9° F., andcontaining 2.96% N₂, 95.47% methane, 1.10% C₂ hydrocarbons, and 0.47%heavier hydrocarbons, is cooled in indirect heat exchanger 2 to atemperature of −252.6° F. and a pressure of 64.50 psia. This cooled,expanded stream is throttled through valve 3 and introduced intodenitrogenation column 5 comprising 6 trays, at a pressure of 18 psia.An overhead vapor stream 10 is withdrawn from the top of the column 5 ata flow rate of 8,122 lbmol/hr, and contains 31.17% N₂, 68.83% methane,and trace amounts of heavier hydrocarbons, at a pressure of 18 psia anda temperature of −261.9° F. Bottoms stream 11 is withdrawn from thecolumn 5 at a flowrate of 117,329 lbmol/hr, a pressure of 19.45 psia, atemperature of −256.8° F., and contains 1.01% N2, 97.32% methane, 1.17%C₂ hydrocarbons, and 0.50% heavier hydrocarbons. First LNG fraction 6 iswithdrawn from the lowest tray of the column at a flow rate of 121,047lbmol/hr, a temperature of −259.7° F., a pressure of 19.74 psia, andcontains 1.56% N₂, 96.81% methane, 1.14% C₂ hydrocarbons, and 0.49%heavier hydrocarbons. This first LNG fraction 6 is passed throughindirect heat exchanger 2 to produce stream 7, which is at a temperatureof −256.8° F., a pressure of 19.45 psia, and has a vapor fraction of3.1%. Stream 7 is returned to column 5 under the lowest tray to provideboilup. This liquefaction process also requires approximately 229 MW ofpower.

Table 1 sets forth data of corresponding streams of the current processwith a recirculation rate of 2.9 and the prior art process in order tomore clearly illustrate the comparison. We first note that therespective feed streams, 104 and 22, and the respective product streams,110 and 11, and 130 and 10, are substantially identical with respect toall relevant properties. This equivalency of feed streams and productstreams enables a valid comparison of the two processes.

As demonstrated in Table 1, a significant difference between the twoprocesses is that the reboiler stream of the current process 112 is at aflow rate of 17,704 lbmol/hr, which is only 14.6% of the flow rate ofthe reboiler stream 6 of the '165 patent process, 121,047 lbmol/hr. Thisdifference is attributable to the fact that, while the '165 patentprocess requires that the entire liquid flow off of a column tray berecycled through the reboiler heat exchanger, the current process may bedesigned to function with various recirculation rates, permittingoptimization of the amount of flow necessary to achieve the desiredseparation, and therefore only recycles the amount of bottoms liquidnecessary to produce the required product. Another noteworthy differencebetween these processes is that, while the total fluid flow through thereboiler is substantially less for the current process than for theprocess of the '165 patent, because the same amount of heat istransferred in each reboiler, a greater percentage of the reboilerstream is vaporized in the current process, 25.3% versus 3.1%. Theamount of vapor actually returned to the column for boilup is thereforegreater for the current process (4479 lbmol/hr), than for the '165patent process (3752 lbmol/hr).

TABLE 1 comparison of the current process (recirculation rate = 2.9) andthe ′165 patent process Stream Current Process The ′165 Patent Processand Flow N₂ CH₃ Temp. Pres. Flow N₂ CH₃ Temp. Pres. Ref. # (lbmol/hr)mol % mol % ° F. psia (lbmol/hr) mol % mol % ° F. psia Feed 125,470 2.9695.47 −242.9 71.78 125,451 2.96 95.47 −243 71.76 104/22 Vapor 8,14131.06 68.94 −261.9 18 8,122 31.17 68.83 −261.9 18 Product 130/10 LNG117,329 1.01 97.31 −256.8 19.45 117,329 1.01 97.32 −256.8 19.45 Product110/11 Reboil 17,704 1.01 97.31 −256.8 19.74 121,047 1.56 96.81 −259.719.74 Input Stream 112/6 Reboil 17,704 1.01 97.31 −252.7 19.45 121,0471.56 96.81 −256.8 19.45 Output (vapor (vapor Stream fraction = fraction= 114/7 25.3%) 3.1%)

Table 2 sets forth data of corresponding streams of the current processwith a recirculation rate of 26.0 and the prior art process in order todemonstrate the flexibility of the current process. We first note thatthe respective feed streams, 104 and 22, and the respective productstreams, 110 and 11, and 130 and 10, are, again, substantially identicalwith respect to all relevant properties. This equivalency of feedstreams and product streams enables a valid comparison of the twoprocesses.

As demonstrated in Table 2, altering the recirculation rate allows forgreat variation in the amount of fluid recycled through the thermosyphonreboiler 106, while maintaining equivalent recovery. The total fluidflow through the reboiler (streams 112 and 6) is much closer in thiscomparison (117,353 lbmol/hr and 121,047 lbmol/hr), as is the percentageof fluid vaporized in the rebolier (3.7% and 3.1%). Thus, the currentprocess has the flexibility to achieve equivalent separation not onlywhen implementing the preferred lower recycle flow rate, but also underflow rate conditions similar to the prior art.

TABLE 2 comparison of the current process (recirculation rate = 26.0)and the ′165 patent process Stream Current Process The ′165 PatentProcess and Flow N₂ CH₃ Temp. Pres. Flow N₂ CH₃ Temp. Pres. Ref. #(lbmol/hr) mol % mol % ° F. psia (lbmol/hr) mol % mol % ° F. psia Feed125,474 2.96 95.47 −242.9 71.84 125,451 2.96 95.47 −243 71.76 104/22Vapor 8,121 31.54 68.46 −262.0 18 8,122 31.17 68.83 −261.9 18 Product130/10 LNG 117,353 0.98 97.34 −256.7 19.45 117,329 1.01 97.32 −256.819.45 Product 110/11 Reboil 117,353 0.98 97.34 −256.7 19.74 121,047 1.5696.81 −259.7 19.74 Input Stream 112/6 Reboil 117,353 0.98 97.34 −254.819.45 121,047 1.56 96.81 −256.8 19.45 Output (vapor (vapor Streamfraction = fraction = 114/7 3.7%) 3.1%)

Although the invention has been described in detail with reference tocertain embodiments, those skilled in the art will recognize that thereare other embodiments within the scope of the claims.

1. A process for the denitrogenation of a liquid natural gas (LNG) feedstream, wherein said LNG stream comprises 1-10 mol % nitrogen,comprising: (a) expanding the LNG feed stream via means provided toexpand the LNG feed stream and cooling the LNG feed stream in athermosyphon reboiler to form a cooled, expanded LNG stream, wherein theexpansion is performed before the cooling or the cooling is performedbefore the expansion; (b) introducing the cooled, expanded LNG streaminto a nitrogen rejection column, comprising a distillation section anda sump; (c) withdrawing a nitrogen-enriched overhead vapor stream fromthe column; (d) withdrawing a first nitrogen-diminished bottoms liquidstream from the column; (e) withdrawing a second nitrogen-diminishedbottoms liquid stream from the column; (f) at least partially vaporizingthe second bottoms liquid stream by passing said second bottoms liquidstream through the thermosyphon reboiler of step (a); and (g) injectingthe partially vaporized second stream of step (f) into the column at aposition of the column above that from which the second bottoms streamof step (e) is withdrawn and below that which received the LNG feedstream of step (b), to provide boilup for the column.
 2. The process ofclaim 1, wherein the first nitrogen-diminished bottoms liquid stream ofstep (d) and the second nitrogen-diminished bottoms liquid stream ofstep (e) are withdrawn together from the column through the sameconduit, and are divided following their withdrawal.
 3. A process forthe denitrogenation of a liquid natural gas (LNG) feed stream, whereinsaid LNG stream comprises 1-10 mol % nitrogen, comprising: (a) expandingthe LNG feed stream via means provided to expand the LNG feed stream andcooling the LNG feed stream in a thermosyphon reboiler to form a cooled,expanded LNG stream, wherein the expanding is performed before thecooling or the cooling is performed before the expanding; (b)introducing the cooled, expanded LNG stream into a nitrogen rejectioncolumn, comprising a distillation section and a sump; (c) withdrawing anitrogen-enriched overhead vapor stream from the column; (d) withdrawinga first nitrogen-diminished bottoms liquid stream from the column; (e)passing a portion of the liquid in the sump of the column through thethermosyphon reboiler of step (a) to provide boilup for the column;wherein the thermosyphon reboiler is positioned within the sump of thecolumn.
 4. The process of claim 1, wherein the means provided to expandthe LNG feed stream is a dense fluid expander.
 5. The process of claim1, wherein the first nitrogen-diminished bottoms stream of step (d) iscollected as a LNG product.
 6. The process of claim 1, furthercomprising passing the cooled, expanded LNG stream of step (a) through avalve before introducing the cooled, expanded LNG stream into thenitrogen rejection column.
 7. The process of claim 1, wherein theexpanding of step (a) is performed before the cooling of step (a). 8.The process of claim 1, wherein the cooling of step (a) is performedbefore the expanding of step (a).
 9. The process of claim 3, wherein themeans provided to expand the LNG feed stream is a dense fluid expander.10. The process of claim 3, wherein the first nitrogen-diminishedbottoms stream of step (d) is collected as a LNG product.
 11. Theprocess of claim 3, further comprising passing the cooled, expanded LNGstream of step (a) through a valve before introducing the cooled,expanded LNG stream into the nitrogen rejection column.
 12. The processof claim 3, wherein the expanding of step (a) is performed before thecooling of step (a).
 13. The process of claim 3, wherein the cooling ofstep (a) is performed before the expanding of step (a).